Brake Pedal goes to floor

If the seal fails between the front and rear chambers of the MC, that will give you the symptoms that you described. Moving the MC piston as far as you did when bleeding the system may have caused that seal to fail. However, that does not explain why the rears lock up. As others said, it sounds like there is air still in the front system.

Make sure your bleeding technique is right. Have someone sit in the car, open a bleeder and immediately have them push the brake pedal SLOWLY to the floor and then HOLD it there while you close the bleeder. Then have them SLOWLY allow the pedal to come back up to the top.

Your technique of pushing the pistons back in the front calipers is correct. What brand of front brake pads did you use?

When you put on the new MC, make sure you follow the 'bench bleeding' procedure.

The difference between the manual and power brake MC's is the piston diameter.

And it is MANDATORY with a car this old to replace all 3 rubber lines. It is about 90% probable that one or all of them are now starting to have internal failures that you cannot see.